The primary long-term goal of this project is the achievement of a further understanding of red cell formation and certain aspects of bilirubin metabolism in the fetus. The investigation of several aspects of fetal erythropoiesis, among them, specific internal fetal control mechanisms and the placental transfer and roles of the nutritive elements, iron and vitamin B12, will be included. Study of bilirubin metabolism will be concerned particularly with intestinal absorption of conjugated and unconjugated bilirubin. Experiments will be conducted in pregnant dogs, sheep and monkeys. Fetuses will be prepared in utero with vascular catheters and then maintained chronically for periods of days or weeks. Parameters of fetal hematologic function (including rates of erythropoiesis) will be determined in normal fetuses and in fetuses undergoing specific alterations in hematologic status. The placental transport of iron and vitamin B12 will also be investigated in animal preparations of the same type. Fetuses having intestinal and biliary catheters will be used for the bilirubin studies.